This invention relates to a semiconductor emitting device using a II-VI compound semiconductor of low resistivity and a process for preparing the same.
Conventional semiconductor emitting devices, for example, semiconductor lasers, are in a double-hetero junction structure composed of III-V compound semiconductor materials such as In.sub.x Ga.sub.1-x P.sub.y As.sub.1-y /GaAs, Ga.sub.x Al.sub.1-x As/GaAs, In.sub.x Al.sub.1-x P/In.sub.y Ga.sub.1-y P (0&lt;x and y&lt;1), etc. and their emitting wavelength is limited to the infrared region and the visible red region.
On the other hand, semiconductor lasers having an emitting wavelength in visible short-wavelength regions such as the yellowish orange region, the green region and the blue region will have many advantages, if practically utilized. For example, if a blue light semiconductor laser is utilized for an optical disk, the recording density can be increased, and a semiconductor laser of the ultraviolet region over to the green region can make the sensitivity of an optical printer higher. Furthermore, plastic optical fibers, which are regarded as important in short-distance communications, have a high loss in the infrared region and have a low-loss region at about 550 nm. Thus, a green light semiconductor laser has been regarded also as an important light source for short-distance communications. Furthermore, an ultraviolet light semiconductor laser could be applied as a light source for phosphor excitation or as a light source for a process technique using light-sensitive materials or as an experimental light source. Thus, visible light semiconductor lasers having a shorter wavelength than 0.5 .mu.m band have many advantages and their practical applications have been keenly desired. II-VI compound semiconductors such as ZnSSe and ZnSTe have been regarded as blue green and yellowish orange light emitting materials. In order to prepare emitting devices with high efficiency, it is necessary to form n-type and p-type conduction layers with controlled conductivity, that is, low resistivity, particularly a resistivity of not more than 10 .OMEGA.-cm, preferably not more than 1 .OMEGA.-cm. However, in the past it has been found difficult to form a thin film of p-type II-VI compound semiconductor with a low resistivity owing to the self compensation effect or residual impurity.
Group-I elements and group-V elements are regarded as acceptor impurities of II-VI compound semiconductors. Group-I elements can act as acceptors by substitution at the lattice site of group-II elements, and group-V elements can act also as acceptors by substitution at the lattice site of group-VI elements.
Recently, it was disclosed in Appl. Phys. Lett., Vol 52, No. 1, 1988, p. 57 that a p-type ZnSe thin film having a resistivity of 0.2 .OMEGA.-cm, that is, ZnSe doped with Li as group-I element was formed at a substrate temperature of 450.degree. C. by metalorganic vapor phase epitaxy, which will be hereinafter referred to as the MOVPE process.
It was also disclosed in Japanese Patent Application Kokai (Laid-open) No. 63-184373 that a p-type, ZnSSe structure having a carrier concentration of 5.8.times.10.sup.15 cm.sup.-3, doped with Li, was formed at a substrate temperature of 500.degree. C. by MOVPE process.
Furthermore, it was disclosed in Jpn. J. Appl. Phys., Vol. 27, No. 5, 1988. p. L909 that a p-type ZnSe thin film was formed at a substrate temperature of 350.degree. C. by doping with N as group-V element by the MOVPE process. The lowest resistivity obtained was 10.sup.2 .OMEGA.-cm and the carrier concentration was 10.sup.14 cm.sup.-3.
Still furthermore, it was disclosed in Jpn. J. Appl. Phys., Vol. 27, No. 11, 1988, p. L2195 that a p-type ZnSSe thin film having a resistivity of 21.9 .OMEGA.-cm and a carrier concentration of 7.2.times.10.sup.15 cm.sup.-3 was formed at a substrate temperature of 500.degree. C. by doping with N by the MOVPE process.
When group-I elements such as Li and Na are used as acceptor impurities in the above-mentioned prior art, deterioration of electric properties of p-type ZnSSe thus prepared is considerable owing to the large diffusion constants of these atoms in the ZnSSe lattice, and the life of p-n junction prepared therewith is short. There is also a problem in the reproducibility. When these group-I elements are interstitially incorporated, they act as donors on the contrary and thus p-type conductivity control is hard to accomplish.
When N (nitrogen) is used as a dopant, a stable thin film with less time-evolution can be obtained, but it is difficult to make the resistivity lower than 20 .OMEGA.-cm. It is usual to conduct doping with NH.sub.3 in a source molar ratio of group-VI element/group-II element of 5 to 100, that is, by supplying the group-VI source in excess of the group-II element source, and in that case the nitrogen atoms are hard to make substitution at the sites of selenium atoms in the p-type ZnSe thin film and it is also necessary to use such a large amount of group-V nitrogen dopant source as 100 times the amount of group-VI element source for reducing the resistivity approximately to 10.sup.2 -10.sup.3 .OMEGA.-cm. This also causes to deterioration in crystallinity.
As explained above, p-type, II-VI compound semiconductor thin films of low resistivity with less deterioration have not been formed with good reproducibility, and thus semiconductor emitting devices having a p-n junction or a double-hetero junction structure have not been obtained.